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The presence of visual hallucinations (VH) is a significant predictor of dementia in Parkinson's disease (PD) and it is associated with a more rapid cognitive decline. Non-demented PD patients with VH present greater neuropsychological impairment than those without VH in domains such as verbal and visual memory, language comprehension, and visuospatial and visuoperceptive functions. Frontal dysfunction has also been described in PD with VH, including deficits in verbal fluency, sustained attention, and inhibition. In PD with VH, structural and functional abnormalities within the primary visual system and visual association areas, including ventral and dorsal pathways, have been reported. Structural MRI studies have shown that non-demented PD patients with VH present grey matter reduction in parieto-occipital areas and the hippocampal head. A follow-up study performed at a mean of 30 months revealed that unlike PD patients without VH, PD patients with VH frequently develop dementia associated with progressive atrophy in limbic, paralimbic and neocortical areas. Functional MRI (fMRI) studies have revealed altered activation in occipito-temporal and frontal areas in response to simple and complex visual stimuli in PD patients with VH, suggesting a marked impairment in bottom-up visual processing, as well as an attentional deficit in the pathophysiology of VH in PD., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Cognitive dysfunction occurs at early stages of Parkinson's disease (PD). Initial studies reported that cognitive dysfunction in early PD only affected fronto-striatal circuits, provoking a marked executive dysfunction. Memory impairment in PD was thought to depend on a problem in retrieving stored information, therefore also reflecting a fronto-striatal dysfunction. However, there is increasing structural MRI evidence of medial temporal lobe atrophy in PD, which may be responsible for memory dysfunction. Other neuropsychological functions usually impaired in PD are semantic fluency, visuoperceptual and visuospatial functions, decision-making and recognition of facial emotions; and impairments in these functions are associated with cortical structural changes assessed by MRI. Overall, although the literature on the topic is scarce, there is increasing evidence of brain structural changes, detectable by MRI, which can explain the neuropsychological deficits early in the clinical disease course before dementia develops. In this review, we summarize the papers that have used structural MRI to study the neuroanatomical correlates of cognitive dysfunctions in PD., (Copyright © 2011 Elsevier B.V. All rights reserved.)